1. Field of the Invention
This invention pertains to the general field of equipment for burnishing magnetic-disk media in the computer industry. In particular, it provides a new machine for burnishing the magnetic disks of conventional computer diskettes after assembly.
2. Description of the Prior Art
Most of the electronic information generated by personal computer users is recorded on flexible magnetic disks that provide compact and easy-to-use storage and portability. These disks are available commercially in two formats that have become standard within the industry: an older 5.25-inch flexible ("floppy") diskette, and a more recent 3.5-inch hard-shell disk. Although applicable to both formats, this invention is described particularly in terms of the 3.5-inch disks, which are rapidly becoming the prevalent format in the industry. In both formats, a flexible disk of magnetic medium is mounted in a protective cover within which the disk spins during use while maintaining contact with a magnetic read/write head through a window in the cover. The assembly includes a liner sandwiched between the cover and the disk for removing impurities that may become lodged therebetween as the disk rotates.
During the course of manufacture, each magnetic disk is produced by punching it from a polyester sheet substrate coated on both sides with magnetic particles suspended in a binder. The steps of manufacturing and coating the polyester substrate produce surface asperities that during use of the disk may cause a separation between the magnetic head and the disk, which in turn would results in loss of signal amplitude and possible read/write errors. Data errors consist of missing bits and extra bits. Missing bits are the most common and are usually caused by asperities on the surface of the coating or by airborne or process-related particles large enough to cause a separation between the head and the coating.
In order to correct this problem, it is necessary to polish the disks to produce as smooth a surface as possible, preferably to within a 0.1 .mu.m tolerance. Therefore, the magnetic medium is burnished to remove surface asperities prior to assembly into the protective cover; this is typically done by rotating each disk while pressed against a very fine abrasive tape. After burnishing, the disks are then inserted into the protective cover which is sealed for final assembly.
The need for surface improvement of the magnetic medium by mechanical means was recognized early in the development of computer science when it was observed that raw disks showed a reduction of read/write errors after having been used for a period of time. Following a rapid initial improvement, the disk remained substantially unchanged until wearout. Accordingly, since that time numerous burnishing approaches have been taken to improve the surface quality of flexible magnetic disks to speed up the break-in period.
A variety of techniques is used by manufacturers to improve the surface quality of the disks produced commercially. These techniques are normally referred as burnishing, polishing or honing. Most modern burnishing operations employ abrasive tape to polish the disk surface. In all cases, the objective is to remove or reduce the height of surface imperfections. All of the burnishing equipment disclosed in the prior art consists of apparatus for polishing the magnetic medium prior to its assembly into the protective cover.
For example, U.S. Pat. No. 3,943,666 to Dion et al. (1976) describes a process that utilizes a rotating ceramic abrasive cylinder pressed against the magnetic medium of the flexible disk. The disk is mounted on a supporting resilient plate through its center hole and rotated in the direction opposite to the motion of the abrasive cylinder.
U.S. Pat. No. 4,179,852 to Barnett (1979) describes a method for polishing floppy disks with a uniform velocity across the radius of the disk. This is accomplished by overlapping the disk mounted on a rotating supporting plate with a polishing medium mounted on a plate rotating in the opposite direction. The patent shows apparatus for performing the procedure on the disk-shaped magnetic material prior to assembly into its protective shell.
In U.S. Pat. No. 4,330,910 (1982), Schachl et al. describe a device for burnishing the surfaces of magnetic disks consisting of a cylindrical body having a planar polishing surface with at least one groove positioned radially across the surface. The disk is polished by pressing the device against the magnetic material while the disk is rotated.
In U.S. Pat. No. 4,430,782 (1984), Bornhorst et al. disclose a burnishing tool having a plurality of holes in the planar surface that engages the flexible magnetic disk. The polishing action is accomplished through a blade positioned at a 45-degree angle with respect to the direction of motion. A vacuum system draws the disk against the burnishing surface and removes debris from the tool during use.
Other, more modern equipment incorporates the burnishing procedure in the manufacturing process as a step performed on each disk prior to enclosure in the protective shell. The burnishing of magnetic disks creates a very large number of small particles that are partly retained by the abrasive medium and partly deposited on the surface of the disk, requiring cleaning during assembly and during use to avoid errors caused by magnetic discontinuities. After burnishing, each disk is fitted with a hub and assembled into a protective plastic shell which is then normally welded around the disk by an ultrasonic weld operation. The welding process produces vibrations that cause the particles on the disk, which are unavoidably subject to some electrostatic or molecular forces, to aggregate and form sufficiently large asperities to affect the intimate contact between the disk and the magnetic head even after the disk has been polished by the burnishing procedure.
Therefore, manufacturers of burnishing machines have resorted to various operating techniques to minimize the problems associated with particle build-up. For example, Applied Data Communications, Inc., of Tustin, Calif., uses deionized air during the burnishing process to neutralize static charges. Wescorp of Niantic, Conn., produces a burnishing assembly machine that sequentially polishes the magnetic medium, applies a center hub to the burnished disk, and inserts it into a protective shell. By avoiding the stacking of polished disks between stages, the build-up of particles on the magnetic surface is minimized.
As the average unit price of flexible disks falls and the operating margins of manufacturers are squeezed, it becomes increasingly important to improve the assembly process and quality control to reduce manufacturing costs. Magnetic media and shell components are already available at very low cost. At the same time, the burnishing and certification steps have a significant effect on the yield and overall cost of manufacturing disks. All disks must be tested or certified to assure that each disk is error free. Depending on the defect, disks that are rejected during certification are either scrapped or sold for a lower price, which obviously affects significantly the overall cost of production. On the average, about 10-25 percent of all disks produced by the industry is rejected during the certification process.
Therefore, there still exists a need for burnishing equipment and techniques that improve quality control during the manufacturing process and reduce the rate of rejection caused by magnetic-medium surface imperfections. This invention is directed at solving these problems.